The present invention relates to an electronic device, and more particularly, to an image sensor device.
CMOS image sensor devices are used in a wide variety of applications, such as digital still camera (DSC) applications. These devices utilize an array of active pixels or image sensor cells, comprising photodiode elements, to collect photo energy to convert images to streams of digital data.
For DSC applications, high-performance imaging with low crosstalk and noise, providing superior low-light performance is required.
Image sensor devices typically suffer from crosstalk, occurring when radiation over one photodetector device is reflected or refracted within the image sensing pixel. The reflected or refracted radiation is detected by the photodetector device of other pixels, thus causing picture distortion. Crosstalk is measured by providing an opaque mask over a photodetector device array that allows radiation (e.g., light) to enter the IC over only one underlying device. Adjacent device response is then measured and the undesired signal divided by desired signal is calculated and defined as crosstalk. Informal industry requirements for high density image sensor provide crosstalk of less than 50% at an oblique incident angle of 15°.
In FIG. 1, a typical structure 100 for measuring crosstalks of an image sensor device is illustrated in cross section, comprising a semiconductor substrate 101 having an array of photodiodes 120 therein. Each photodiode 120 comprises, for example, an n-type region 124 in a p-type region 122. Each photodiode 120 is separated from other photodiodes by an array of isolation structures 110, such as shallow trench isolation (STI). Thus, an array of pixels is obtained. A layer of metal 140 comprising an opening is formed on dielectric layers 130. The opening defined a pixel region for measuring crosstalks of the image sensor. The pixels convert incoming light 160 and 160′ from a light/image source to electrical signals via the photodiodes 124.
In order to serve miniaturization, pixel size is decreased and a multilevel interconnect structure is employed. For example, the substrate 101 is covered by a series of dielectric layers 130, such as an interlevel dielectric (ILD) layer and intermetal dielectric (IMD) layers. As pixel width is scaled down, however, the thickness of the dielectric layers, i.e. vertical dimension of the pixel, remains the same. As a result, incident light 160 and 160′ strikes the surface of the topmost dielectric layer 130. This light is then transmitted through the underlying dielectric layers 130 to the underlying pixels. It is not uncommon for incident light 160 and 160′ to strike the surface of the photodiode device at a variety of angles θ. For example, the light 160 can strike the surface at a near perpendicular angle, and light 160′ at an oblique incident angle.
Light 160, striking the surface at a near perpendicular angle, is transmitted to a photodiode 120I (a pixel) underlying the contact location. This is optimal for image sensing performance. However, light 160′ striking the surface at oblique angle θ may then be transmitted to a nearby photodiode 120II rather than to the pixel 120I directly underlying the contact surface. This is crosstalk. The crosstalk problem can cause degraded image resolution for black and white sensors or complicate color correction for color sensors.
FIG. 2 illustrates curves of crosstalk versus incident angle of conventional CMOS image sensors of different pixel widths. Crosstalk is critical at oblique angles of approximately ±15°. As pixel width is reduced, crosstalk of curves III and IV exceeds 50% at incident angle of ±15°.
It is beneficial to further reduce crosstalk when pixel width is scaled below 3 μm.